Multicomponent seismic data, combining P-wave and converted P-to-SV wave (C-wave) wavefields, provide independent measurements of rock and fluid properties. Unlike P waves, C waves are minimally affected by changes in pore fluids, and in cases of azimuthal anisotropy, will be split into two modes (fast and slow) with differing polarization. The 4C, 3D ocean-bottom cable (OBC) multicomponent seismic data discussed here were acquired in shallow water (<300 ft) offshore Louisiana over approximately 455 miles2 (Figure 1). Because these data are still being marketed to interested oil and gas operators, only data above targeted oil and gas reservoirs (<5000 ft) were used. The P-wave migrated data extend to only 1 s and the C-wave migrated data volume to only 2 s.

Figure 1.

Location of 4C, 3D OBC survey.

The initial objectives of the survey were to improve P-wave reflection quality by combining hydrophone and vertical-geophone data and to improve structural interpretation in the presence of “gas clouds” with C-wave data. Our additional research objectives were to evaluate seismic attributes, such as VP/VS velocity ratios and Poisson's ratio derived from P-wave and C-wave data, for characterizing shallow seafloor sediments and to develop practical poststack 3D data interpretation methodologies to capitalize on these unique characteristics. These research findings will be used later to analyze seafloor properties across gas hydrate prospects. Gas (methane) hydrates in the northern Gulf of Mexico (GOM) typically are not manifested by the classic bottom simulating reflector (BSR) and seem always associated with P-wave data “wipeouts” in gas clouds (verbal communication, Harry Roberts, Louisiana State University). The inability of conventional P-wave surveys to image the internal stratigraphy of gas hydrate systems in the GOM warrants further study. Diagnostic multicomponent seismic attributes that locate shallow hazard zones just above marine gas-hydrate deposits can optimize well placement strategy …